It is widely known to use electrolysis for treatment of different kinds of waste waters, which contain solid or liquid impurities in suspension, emulsion or in soluble form, which impurities are to be separated from the water as a separate phase by means of chemicals produced by electrolysis and/or flotation by electrolytically produced gas bubbles. Various wastes, such as various suspensions from pulp and paper manufacturing industry, oil emulsions and municipal waste waters can be subject to such a treatment, and these processes are widely described in the patent literature, e.g. in U.S. Pat. No. 3,505,188 and in CH-patent 425 655.
The waste water often contains also harmful dissolved substances, which are to be made innocuous by decomposing such substances. This can be done in electrolysis by producing oxygen and/or chlorine, which oxidize organic material present in the water. One of such substances is cyanide, which is present in waste waters resulting from various industrial processes, such as metal plating and refining of gold and silver ores. The technique used conventionally in destroying cyanide includes the adding of NaCl to waste water containing cyanide and passing it thereafter through an electrolytic cell, wherein the chlorine and caustic are generated from the salt and react with the metal-cyanide compound producing harmless precipitates and CO.sub.2 and N.sub.2. Another alternative is to pass a pure electrolyte containing NaCl through an electrolytic cell and to lead it thereafter to a reservoir containing the cyanide waste, wherein the decomposition reaction takes place. These methods are described e.g. in U.S. Pat. No. 4,029,557, GB-patent 1 433 858 and DE-patent application 2 331 189.
One problem in the electrolytic purification processes is that the surfaces of the electrode plates are not utilized optimally. In a flow between the electrodes, a diffusion layer is generated in the proximity of each electrode, which makes the diffusion of reactants onto the operative surface and diffusion of the products therefrom difficult. Moreover, gases generated by the electrolysis reduce the operative surface of the electrodes and they must thus be quickly removed from the surface. Due to the nature of the waste waters, fouling of the operative surfaces by the depositing impurities of the waste water is also a considerable problem.
There have been attempts to overcome these disadvantages by agitating the liquid before it is introduced into the passage between the plates. The conditions between the electrode plates are, however, not controlled sufficiently by this procedure, in particular as the passages between the plates are long and narrow, which is advantageous for the constructional simplicity and the efficiency of the electric current.